A conventional gas-liquid separation device is publicly known from the following Patent Publication 1 in which two mounting seats for mounting a breather case of a breather device having a gas-liquid separation function are provided on a ceiling wall and a peripheral wall of a crankcase of an engine, respectively, and the breather case is mounted on one of the two mounting seats which receives less oil droplets depending on the usage of the engine.
Patent Publication 1: Japanese Utility Model Publication No. 62-12820